Remove documentation for Firefox < 68

Author: juj

test/common.py

@@ -116,9 +116,6 @@ def configure(data_dir):
     """Chrome has no special configuration step."""
 
 
-# N.b. The earliest Firefox version that can run multithreaded browser harness
-# is Firefox 67. If you need to test earlier Firefox versions, use EMTEST_CORES=1
-# environment variable to revert to the single-threaded browser harness.
 class FirefoxConfig:
   data_dir_flag = '-profile '
   default_flags = ('-new-instance',)